The present invention relates to an image projector, more particularly to an image projector with optical valves such as liquid-crystal valves, which is used to display a computer or video image on an external screen.
One example of an image projector using liquid-crystal optical valves is shown in FIG. 1. This projector comprises an illumination system 1 having a lamp 10, such as a metal halide, xenon or similar lamp, and a reflector 11 surrounding the lamp 10. The illumination system 10 is combined with an optical integrator 2, the role of which is to uniformly distribute the light emitted by the lamp 10 of the illumination system, over the components of the optical unit 3 which will be described hereinafter. The optical unit 3 has, schematically, colour separation means so as to supply each optical valve 30, 31, 32 with the corresponding colour component and a means 20 of recombining the images supplied by the three optical valves. As shown in FIG. 1, the image recombination means is formed by a cube 20 constituting a set of mirrors 22 and 24 occupying the diagonal planes of the cube 20. The three liquid-crystal valves 30, 31, 32 are placed parallel to three faces of the cube. More accurately, the optical valve 30 is parallel to the face 26 of the cube. The optical valve 32 is parallel to the opposite face 28 of the cube, the two valves 30 and 32 being parallel to each other and the optical valve 31 is parallel to the face 27 of the cube and perpendicular to the other two valves 30 and 32.
As the illumination system 1 emits white light, colour separation means are therefore provided in the optical system 3. These means are formed by dichroic mirrors combined with plane mirrors in order to direct, respectively, the blue colour component onto the optical valve 30, the green colour component onto the optical valve 31 and the red colour component onto the optical valve 32. More specifically, the light emitted by the illumination system 1 passes through a first dichroic mirror 33 placed immediately downstream of the integrator 2. The normal N to the plane of the mirror 33 forms an angle of 45xc2x0 with the optical axis O. Because of this, the dichroic mirror 33 transmits the blue colour component to a plane mirror 34 positioned so as to illuminate the optical valve 30 via a field lens 40 and reflects the remainder of the spectrum towards a second dichroic mirror 35 parallel to the first dichroic mirror 33. This second dichroic mirror 35 reflects the green colour component of the spectrum towards the optical valve 31 through a field lens 41 similar to the lens 40. The dichroic mirror 35 transmits the remainder of the spectrum, i.e. the red colour component. This red component passes through a first lens 42 then is reflected by a plane mirror 36 parallel to the dichroic mirrors 38 and 40. The component reflected by the plane mirror 36 passes through a lens 44 then is reflected again by a plane mirror 37 in such as way as to be sent back to the optical valve 32 through a field lens 43 identical to the lenses 40 and 41. The images formed on the optical valves 30, 31 and 32 are then recombined by reflection and transmission inside the cube 20 so as to obtain a synthetic image on the face 29 of the cube, this image being sent to a projection lens 5.
The apparatus described with reference to FIG. 1 comprises numerous components, especially optical and electronic components, which have to operate in a clean environment and at acceptable temperature levels. Now, in order to obtain good quality images having a high luminosity, it is generally necessary to use powerful projection lamps. The use of bright lamps which are more and more powerful makes the thermal aspects more critical within the projector. This leads to the use of more and more elaborate cooling systems which must also take into account parameters such as the increase in the resolution of the optical valves, the desire to have the minimum overall size, the noise level of ventilation systems and the increase in the functions offered.
The object of the present invention is therefore to provide an image projector with an improved cooling system which especially enables the following advantages to be obtained, viz.:
to ensure that all the critical parts are kept below their limit temperature;
to allow the proper operation of the projector up to ambient temperatures which may reach 50xc2x0 C.;
to ensure the system is placed away from dust and to minimize as far as possible the noise level.
As a consequence, the subject of the present invention is an image projector comprising a casing containing at least:
an illumination system;
an optical unit comprising at least one optical valve modulating the light emitted by the illumination system;
a projection lens projecting the modulated light outside the casing;
a power supply, control electronics and cooling means;
the casing being provided with at least one aperture forming an air inlet,
characterized in that it further comprises a specific air filtration means positioned under the optical unit and in such a way as to surround the latter.
According to a preferred embodiment, the air filtration means is a component in the shape of a cage, the side walls of which are covered with wire mesh.
Preferably, the present invention is applicable to a colour image projector the optical unit of which comprises three optical valves each provided for one colour, colour separation means to supply each optical valve with the corresponding colour component from the light emitted by the illumination system and a means of recombining images supplied by the three optical valves.
According to another characteristic of the present invention, the projector further comprises at least one cooling means for the optical valve or valves. Preferably, it comprises three turbines positioned inside the specific air filtration means, under each optical valve respectively.
To obtain better cooling of the projector, it further comprises a cooling means for the illumination system formed by a turbine positioned horizontally close to the illumination system, this turbine being extended by two air guides, a first air guide sending part of the air to the illumination system and a second air guide sending the other part of the air to the optical unit.
Moreover, the projector may comprise additional cooling means and air extraction means generally formed by fans.
According to an additional characteristic of the present invention, the speed of the turbines and of the fans is variable according to the temperature, the temperature being detected by at least one thermistor combined with an electrical control circuit for each fan.
Moreover, according to yet another characteristic of the present invention, the casing has an aperture forming an air inlet on each side face and at least one air extraction aperture on the rear face.